I’d definitely say bottom left if you’re going up against someone like a Roman hoplite, or similarly shielded but not so armored opponent. It doesn’t take too much force to fuck up an unarmored arm, which a flail vs shield would be perfect for.

If they’re shielded and armored, though, I don’t think a flail is going to be so great.

Though as with all fighting, if one person is far more skilled than the other, they’re winning most of the time, regardless of (competent) armaments.